(I) Field of the Invention
This invention relates to the production of a novel type of block copolymer, and more particularly to a block copolymer whose molecule contains hydrophilic and hydrophobic blocks in a controlled amount. The invention also relates to a method of manufacturing such a block copolymer and bio-compatible materials comprising such a block copolymer.
(II) Description of the Prior Art
A polymeric material having both hydrophilic and hydrophobic properties is known as an amphiphilic polymer exhibiting a good bio-compatibility. Such polymers known to date are poly (hydroxy acrylate) and poly (hydroxy methacrylate). The typical one of such polymers is poly (2-hydroxyethyl methacrylate) commercially known as "Hydron". This substance has both hydrophilic and hydrophobic properties, because the molecule contains hydrophobic .alpha.-methyl groups and backbone (main chain), and hydrophilic hydroxyl groups located apart from the main chain. Hydron which exhibits bio-compatibility due to the above-mentioned characteristic has been attracting attention as a hopeful material particularly in a biomedical field.
However, the poly (hydroxy methacrylate) has the drawbacks that the presence of a diester inevitably contained in the starting monomer (hydroxy methacrylate) leads to the formation of a cross-linked polymer during polymerization. Thus, great difficulties are encountered in producing a linear polymer.
The customary method of manufacturing the above-mentioned poly (hydroxy methacrylate) experienced considerable difficulties in designing a molecular structure suitable for a demanded function and controlling the length of a molecular chain. As a result, poly (hydroxy methacrylate) obtained by the conventional process has been restricted in application. Development is being made for the application of the above-mentioned three-dimensional polymer to the biomedical field as a contact lens or catheter and many other fields on the basis of the water-swellability of the polymer. However, this three-dimensional polymer has a low mechanical strength, and insufficient water-swellability. Moreover, the molecules of the polymer have a low mutual adhesivity. At present, therefore, the polymer is practically applied only in a limited field.
In addition, the aforesaid three-dimensional polymer exhibits a prominent hydrophobic property due to the presence of the .alpha.-methyl groups of the side chain and the main chain or backbone and consequently is restricted in the affinity to water, though the side chain contains a hydrophilic hydroxyl group.
Generally, the important requirements for a polymer to have bio-compatibility are not only the possession of both hydrophilic and hydrophobic properties by the polymer but also the formation of microphase separated structure in which hydrophilic and hydrophobic blocks are independently aggregated and distributed. Studies conducted to date show that a biomembrance involving vascular endothelium are supposedly formed of both hydrophilic and hydrophobic domains, constituting a structure in which the microdomains are separated from each other in the level of assembly state of polymer. Thus, such structure is very important for a polymer to have bio-compatibility.
From the above-mentioned point of view, the aforesaid poly (hydroxy methacrylate) is not considered preferable. The reason for this is as follows. The hydrophilic and hydrophobic properties of poly (hydroxy methacrylate) result from the corresponding groups of the monomer in the molecular level. Therefore, the polymer itself can not be considered to have a microstructure in which microdomains are separated from each other in the level of assembly state of polymer.
From the above-mentioned circumstances, a block copolymer has come to be highlighted which is formed of different hydrophilic and hydrophobic chains bonded to each other in the polymer molecule. Due to the recent development of the so-called living anion polymerization technique, it has become possible to synthesize block copolymers having various structures with high monodispersity.
Where a monomer such as styrene is polymerized, for example, in anhydrous tetrahydrofuran with naphthyl sodium used as a polymerization initiator, then a living polymer is produced which has a stable active site (in this case, carbanion) at both chain ends and is free from termination reaction and chain transfer reaction. Addition of another monomer to the living polymer provides a block copolymer.
However, the above-mentioned method based on the living anion polymerization technique has the drawback that where styrene is replaced by hydroxy methacrylate containing a polar group such as hydroxyl group, then chain transfer reaction takes place to suppress the activity of a living anion and consequently stop the growth of molecules. In other words, the living anion polymerization technique is encountered with considerable difficulties in producing a block copolymer from hydroxyalkyl acrylate or hydroxyalkyl methacrylate.